Introduction
Driven by environmental concerns, resource optimisation goals, costs abatement, and market competitivity, the aquaculture sector is not exempt from seeking circular and less impacting solutions. By definition, RAS systems allow for direct recirculation of water. Here, a step forward is analysed, addressing by-products that are currently not recirculated in the plant nor in outer human economies, in turn implying safety and environmental threats in addition to disposal expenditures: namely, wastewater and fish mortality. The former yields sludge that is converted into biogas, while the recirculated water still averagely contains relevant quantities of nitrogen (N, 0.028 mg/L), ammonium (NH4, 0.036 mg/L), Chemical Oxygen Demand (COD, 219 mg/L), and suspended solids (SS, 100 mg/L) when leaving a regular RAS. The latter is mostly treated by the technology of ensilage, with biomass treated with formic acid, thus causing a hazardous by-product to be transported and safely disposed of, while exposing humans to a liquid that is able to cause acid etching to the skin, eyes, lungs, and more; the waste liquid may also produce gases that are harmful to human health and that can be explosive. Targeting such two by-products, some eco-innovations were developed within the Horizon 2020 project GAIN – Green Aquaculture Intensification in Europe (2018–2021), aimed at abating costs and impacts for the farm, and valorising current by-product as new resources for fertiliser mixes, pet food, cement production and/or energy generation. Carried out by Norwegian industrial partners Salten Havbrukspark (SHP) and Waister AS (WAS), such proposals were independently evaluated by Italian public research partner Università Ca’ Foscari Venezia through an emerging comprehensive environmental accounting method – i.e. EMergy assessment – allowing to enlarge the evaluation boundary compared to well-oiled Life-Cycle Assessment (LCA), which is anyway also used here for comparisons.
Materials and methods
This study presents the environmental assessment of several options per innovation. In both, scenarios (A) represent reference conditions depicting business-as-usual approaches. Regarding wastewater, innovative scenario SHP(B) shows the consequences of a filtered and dried sludge, i.e. more purified water to be recirculated and a resulting powder, rich in nutrients, to be recirculated and valorised in outer economies: (B1) as a fertiliser; (B2) as bio-energy in a cement factory; and (B3) as biogas substrate. Regarding fish mortalities, drying innovations differ for the cooling media of the new machinery, i.e. water in WAS(B) and a mix of water and glycol in WAS(C), and for the consequent fact that in (B) hot water is recirculated in the RAS, allowing to save electricity in the fish farm. For such innovation, end-of-life valorisation options are represented by the reuse of the dried product as a pet food ingredient, as bio-energy in a cement factory, and as biogas substrate. Both eco-innovations are referred to demonstration plants in Norway; all inputs, including transportation ones to the final valorisation sites, are computed for that geo-economic context, yet allowing for exportation and adaptation. The economic implications of savings from the avoided disposal or even by the sale of by-products are also evaluated in the present assessment. Indeed, this is possible through the thermonidamics- and systems ecology-based EMergy accounting (EMA) approach (Odum, 1996; Brown & Ulgiati, 2016): changing the perspective from the final user (receiver) to the environment (donor), material and immaterial inputs and outputs are associated with their direct and indirect requirements to the geobiosphere. LCA results from the standardised Life-Cycle Assessment (LCA) (Arvanitoyannis, 2008) are also offered and discussed. Through EMA, the reliance upon natural resources is further explored compared to LCA, focusing more on impacts. Such approaches may be seen as complementary; for this reason, they are increasingly used together for more comprehensive views on a process’ sustainability.
Results
The eco-innovations’ potentials, limits, and margins for improvement are presented and discussed. The eco-innovations for fish mortality treatment seem to perform better than business-as-usual ensilage. Through LCA, environmental gains larger than –80% are obtained in most indicators, even when the product is simply disposed of, and larger environmental gains arise from the reuse of the dried product in the processing of pet food, implying avoided disposal and savings in alternate ingredients. Positive LCA results, although smaller than in the other innovation, also come from innovations addressing wastewater, with no marked preference among all end-of-life options. In both innovations, smaller, neutral, and in some cases contrasting results are reached on water consumption. Compatible trends seem to be confirmed by EMA, anyway still in progress while submitting the present proposal, allowing for further insights about the mineral and fossil fuel scarcity, and adding up novel information about flows that are instead not accounted for in LCA: among these, the most interesting ones for the assessment at hand seem to derive from free-of-charge renewable inputs, from human labour, and from the environmental significance of the monetary costs associated with the purchase and selling of inputs, waste, and valorised by-products.
Acknowledgements
The research leading to these results has received funding from the European Union’s HORIZON 2020 Framework Programme under Grant Agreement no. 773330.
References
Arvanitoyannis, I. S. (2008). ISO 14040: life cycle assessment (LCA)–principles and guidelines. Waste management for the food industries, 97-132.
Brown, M. T., & Ulgiati, S. (2016). Emergy assessment of global renewable sources. Ecological Modelling, 339, 148-156.
Odum, H. T. (1996). Environmental accounting: emergy and environmental decision making. Wiley.